1. Field of the Invention:
The present invention relates to a full-wave rectifying device. More specifically this invention relates to a transistor full-wave rectifying device.
2. Description of Related Art:
Radio-Frequency Identification (RFID) System is an automatic identification method that involves affixing a small electronic tag to a product which may be checked and monitored by a device known as “reader” which in turn transmits the data stored in the electronic tag back to the system via a wireless RF means, thus achieving remote authentication, tracking, control, management and handling.
The electronic tags are categorized into two general varieties, passive and active. In particular, passive RFID tags have no internal power supply. The minute electrical current induced in the antenna by the incoming radio-frequency (RF) signal provides just enough power for the CMOS integrated circuit in the tag to power up and transmit a response.
Generally, the amplitude of the RF signal is approximately 200 mV or lower, and it is not easy to power up passive RFID tags with such a weak signal. Moreover, current leakage or parasitic effect often occurs in electrical circuits, thereby causing the RF signal energy to be further dissipated.
Consequently, a rectifier is designed such that, upon receiving the RF signal by the antenna of passive RFID tags, the capacitors are charged repeatedly, thereby converting the RF signal to a sufficient dc voltage level for the next stage of circuit operation.
Referring to FIG. 1, the first conventional full-wave rectifying circuit structure is shown. The full-wave rectifying circuit has eight transistors, P1-P6, N1 and N2. Such a circuit structure lowers the transistor body effect, thus improving the efficiency of converting the RF signal to a dc voltage.
However, such a circuit structure is only a single-stage rectifying circuit, which produces the dc voltage output that is approximately the amplitude of the RF signal only. Even under the circumstance of 100% conversion efficiency, such a circuit structure usually fails to provide a sufficient dc voltage level.
Referring to FIG. 2, another conventional full wave rectifying circuit structure is illustrated. Diode-connected transistor instead of Schottky diode is used in the cascade of a multistage rectifying circuit, thereby deriving a sufficient dc voltage level given a specified input power.
However, the chip manufacturing process of such a circuit structure does not take into account the importance of separate bulk connection which suppresses the body effect. Hence, such a circuit may be affected by the body effect, thus causing a significant difference in the threshold voltage of the transistor at each stage. Consequently, the output voltage level is lowered and the circuit fails to output a dc voltage level that matches the designed value.
In summary, it has become an urgent issue to designers of the RF circuit design field to propose a multistage full-wave rectifying circuit that not only provides a sufficient dc voltage level, but also avoids or lowers the body effect such that the difference in the threshold voltage of the transistor at each stage is reduced to minimum, in order for the circuit successfully to output a dc voltage level that matches the designed value.